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Energies 2020, 13, 420 7 of 96 Similarly, syngas could be obtained by introducing a mixture of CO2 and H2O in the reactor vessel. CO2/H2O thermochemical cycles are a possible route for the utilization of the carbon dioxide captured from other processes. Low-temperature oxidation with CO2 (CO generation step) MOred + CO2 → MOox + CO (4) The previous reactions may be classified as oxide type and are the most investigate nowadays, but other two types of reaction are developed: the hydride type and the hydroxide type [81]. Hydridetype: MH2 → M+H2 (5) M + H2O → MH2 + 12 O2 (6) Hydroxide type : 2MOH → 2M + H2O + 21 O2 (7) 2M+2H2O → 2MOH+H2 (8) Many thermochemical cycles with metal oxide redox couple have been proposed [82], including Fe3O4/FeO [83–86], TiO2/TiOx [87], Mn3O4/MnO [88], Co3O4/CoO [89,90], ZnO/Zn [86,91,92], SnO2/SnO [93–95], CeO2/Ce2O3 [96,97], CdO/Cd [98,99] and W/WO3 [100]. The most investigated materials are zinc, iron and ceria metal oxides. Nakamura [101], in 1977, first proposed a two-step thermochemical cycle based on Fe3O4/FeO. Iron oxide has a great reduction capacity that enhances the product yield, but the high volatility of ferrite oxides reduces the lifetime of the material and increase operational cost. Ceria is more stable than ferrite but requires a higher temperature in the activation step [102]. Zinc is probably the most promising metal oxide redox pair for its high energy efficiency (44% with heat recovery from the quenching process and 29% without the heat recovery) [86,91]. The use of mixed metal oxides could decrease the reduction temperature required and increase the melting point. Mixed metal oxides are generated by the partial substitution of the primary materials with dopants including Mn, Co, Ni, YSZ and Zn for ferrite cycles [103–108], and Zr, Hf, La, Pr, Sm, Gd and Tb for ceria cycles [109–113]. The theoretical energy conversion efficiency of the iron oxide cycle is 39% and up to 75% by recuperating the high-temperature heat and 28% for un-doped ceria without heat recovery [114,115]. Some of the redox pairs, when reduced, mutate to the gaseous phase (for example, the reduction of ZnO and In2O3 to gaseous Zn and In2O, respectively) [80,81]. Volatile cycles are thermodynamically more advantageous, since the formation of the vapor product ensures a higher entropic gain. However, a quenching reactor is needed after the activation step to avoid possible recombination with oxygen and to recover the reduced metal oxide in the condensate phase [116,117]. Non-volatile cycles allow the continuous removal of oxygen from the solid metal oxide, avoiding the recombination and quenching problems. Three-step thermochemical water splitting is similar to two-step thermochemical cycles, but the former generally requires a lower maximum temperature. However, three-step cycles involve a higher complexity of the plant. Reactions involved in a three-step metal oxide cycle are: MOox → MOred + 12 O2 (9) MO +2M′OH→ M′ O·MO +H (10) red 2 ox 2 M′ O·MO +H O→MO +2M′OH (11) 2 ox 2 ox In the first step, the metal oxide (MOox) is thermally decomposed to the metal form or the lower-valence metal oxide (MOred), and oxygen is released. In the second step, the reducedPDF Image | Green Synthetic Fuels
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